There is increasing interest in implementing broadband communication systems on various forms of mobile platforms, for example, maritime vessels. With a broadband satellite communication system that has an antenna mounted on a maritime vessel (for example, a tanker, freighter, passenger ferry, etc.), the antenna is used to help form a communications link with a space-based satellite in geosynchronous orbit. The antenna forms part of a communications terminal that is carried by the vessel.
With such systems, maintaining closure of the communications link between the antenna and the satellite depends upon an unobstructed and uninterrupted line of sight between the vessel-mounted antenna and the satellite. However, this requirement for an unobstructed line of sight between the vessel antenna and the satellite is rarely completely satisfied for any vessel installation. This is because of intermittent obstruction of the line-of-sight path by other portions of the vessel as the vessel travels. This problem is compounded by the number of antennas and tower-mounted components that are typically used on maritime vessels. Such obstructions may be caused by various fixed objects (for example, a tower or wall) that interferes with the line-of-sight path to the satellite. Intermittent line-of-sight obstruction can occur when the vessel rolls into the view of the antenna because the antenna is space stabilized, whereas the vessel is not. In this instance the obstruction would be temporary.
Obstruction with the line of sight can be partial or complete. In FIG. 1, a front view of a vessel indicates that the broadband satellite antenna may experience at least partial blockage by the Inmarsat C antenna supporting structure, the Inmarsat B antenna, or other components (not shown), such as a radar antenna or various other components supported above the vessel's bridge. The degree of blockage caused by any one of the above structures with the line of sight will depend upon numerous factors, including the location and/or heading of the vessel, the pitch, roll or yaw of the vessel, and the azimuth and elevation pointing angles being used to point the broadband antenna at the satellite. Depending upon the route of the vessel, one or more of the above-described structures may periodically partially or completely interfere with the line of sight between the broadband antenna and an orbiting satellite.
FIG. 2 presents a simplified diagram of the obstructed field in the azimuth plane caused by a given obstruction, for example, the stage of the vessel in FIG. 1. FIG. 3 illustrates how the degree of the obstructed field of view in FIG. 2 is significantly reduced simply by increasing the distance D3>D2 separating the broadband antenna and the obstructing component.
When several potentially obstructing objects are factored in, the broadband antenna may suffer varying degrees of blockage depending on its position. This is illustrated in FIGS. 4-6. FIG. 4 illustrates an azimuth view for the areas of blockage of a single broadband antenna for two different positions. FIG. 5 illustrates a azimuth coverage diagram indicating the areas of coverage and blockage for position 1. FIG. 6 illustrates the areas of azimuth coverage and blockage for position 2.
One method that could be employed to eliminate the line-of-sight blockages is by locating the broadband antenna well above the highest point on the vessel. However, this is not always possible. Moreover, locating the broadband antenna at the highest point of the vessel may cause the broadband antenna itself to interfere with the line of sight of other antennas or lights on the vessel (for example, radar antennas or the Inmarsat B antenna) that may be performing critical navigation and/or mission functions.
Accordingly, it would be highly desirable to provide a means for moving the broadband antenna between two or more positions when it is determined that a line of sight between the antenna and a satellite in communication with the antenna is being partially blocked, so as to affect the quality of the communications link between the antenna and the satellite.
It would also be highly desirable if such a system can be used to predict when partial or complete blockages will be (or are) occurring, and to determine which one of two or more different antenna positions will provide optimum performance for the broadband antenna (and minimize blockage of other antennas aboard the same vessel). Such a system and method would also enable greater flexibility with regard to the installation of other antennas or components on the vessel, the positions of which would otherwise have to be carefully considered for blockage by the satellite antenna with regard to the routes that the vessel is expected to travel. Such a system and method would also eliminate the need for multiple antennas located at different positions on the vessel to achieve continuous closure of the communications link in view of the blockages that are likely to occur during travel of the vessel, or even while the vessel is stationary at anchor or at a port. It should be noted that blockages can arise in port due to external structures (e.g., a bridge or a building, or even an airplane) that are not part of the vessel's on-board blockage structures. These blockages may also be addressed by the present invention.